This invention relates to a step-by-step motor unit suitable, inter alia, for use in an electronic watch having an analogue display, and more particularly to a step-by-step motor unit that enables increased rotational speed in both directions.
By motor unit is meant not only the motor as such, with its rotor and stator, but also the associated control circuit that generates the drive pulses that need to be applied to the motor depending on the required mode of operation.
In the case of a watch having a seconds-hand driven by a step-by-step motor and which correction is carried out mechanically, it suffices to have a motor which performs a number of steps per second equal to the number of steps that the seconds-hand must perform to move from one graduation to the next. This number is mostly less than six and sometimes equal to one. This means that the motor need only perform, at most, six steps per second. There is no problem in achieving such a speed of rotation for the motor.
However, in the case of a watch without a seconds-hand and in which corrections or time-zone changes need to be made by driving the motor the situation is different. The motor must be drivable both forward and in reverse with a considerably greater frequency, i.e. with a considerably greater number of steps per second. At present, with Lavet-type step-by-step motors, it is impossible to achieve satisfactory operation of the motors at frequencies greater than 50 Hz in the forward direction and 30 Hz in the reverse direction. In the case of a watch having a minutes-hand and an hours-hand and for which two motor steps are necessary for the minutes-hand to move forward by one graduation and assuming that the time-zone is to be moved back by five hours, roughly fourteen seconds will be needed to carry out this one correction. This is hardly acceptable to the user of the watch.
The advantage of having a motor that can be driven at high rotational speed thus becomes apparent.
For a better understanding of the problem reference will be made to FIGS. 1a and 1b of the accompanying drawings. FIG. 1a shows a conventional Lavet-type motor. The motor comprises a rotor 2 consisting of a permanent magnet having a North pole and a South pole, and a stator 4 fitted with a coil 6 receiving current drive pulses. Stator 4 involves two polar portions I and II linked by isthmuses 12 and 12' between which is provided an opening 8 housing the rotor 2. Opening 8 has, for example, two notches 10 and 10' that define a static equilibrium axis x'x for the rotor. Axis x'x defines, for the rotor, two rest positions that are 180.degree. apart; the rotor spontaneously positions itself in one or other of these positions when no current flows in coil 6. FIG. 1a also shows an axis y'y corresponding to the two dynamic equilibrium positions of the rotor, i.e. the axis along which the South-North axis of the magnet tends to position itself when there is a current flowing in coil 6 and when, therefore, a magnetic field is created between the two polar portions I and II of the stator. For the sake of simplicity it will be assumed that the axis y'y coincides with the principal direction of the magnetic flux generated through the motor by coil 6 when the current flows through the latter. In fact, there is a slight angular difference between the two, due to the positioning torque brought about by the notches 10 and 10' and to which the rotor is permanently subjected. Angle .alpha..sub.1 in FIG. 1a between axis x'x and axis y'y can range from 30.degree. to 60.degree., the supplementary angle .alpha..sub.2 ranging between 120.degree. and 150.degree..
The direction of rotation F in FIG. 1 will be regarded as being the positive direction of rotation or forward operation of the motor, the opposite direction corresponding to reverse. Furthermore, a current pulse applied to coil 6 will be deemed positive if it creates a North pole in polar portion I and a South pole in polar portion II, and the pulse will be deemed negative in the opposite case.
Assuming that rotor 2 is in the position shown in FIG. 1a, i.e. the position for which the S-N axis of the magnet corresponds to the axis x'x, and assuming that a positive current pulse is applied to coil 6, the rotor will rotate in the positive direction of angle .alpha..sub.2 until the S-N axis of the rotor coincides with the dynamic equilibrium axis y'y. When the pulse ceases, the rotor spontaneously settles in one of the two positions of static equilibrium on axis x'x, but reversed with respect to the initial position. To cause the rotor to perform another half-turn, i.e. the next step, a negative pulse is applied, setting up an N pole in portion II and an S pole in portion I. Thus, there is no problem in obtaining forward rotation, provided the rotation frequency is not too great: roughly 50 Hz at the most.
At such frequencies the time that elapses between the application of two drive pulses is sufficient compared to the duration of the drive pulse, to ensure that by the time a new pulse is applied the rotor will have reached its position of static equilibrium.
But if the motor is to rotate in reverse from that same initial position, a single negative pulse will not be sufficient because angle .alpha..sub.1 between the axes of static and dynamic equilibrium is too small.
U.S. Pat. No. 4 112 671 describes reverse drive means for such a motor. These drive means are briefly described with reference to FIG. 1b. First, a brief positive pulse 14 is applied which causes the rotor to rotate in the positive direction by a slight angle. Then a negative pulse 16 is applied which actually corresponds to a reverse rotation. The positive pulse 14 causes the angle .alpha..sub.1 to be artificially increased such that the rotor may reach a dynamic equilibrium position with sufficient mechanical energy to enable the next positive pulse 18 effectively to bring it to a static equilibrium position which is phase shifted by .pi. with respect to that it occupied initially. To perform the second step, a series of pulses 14', 16' and 18' are applied, identical to pulses 14, 16 and 18, but of opposite polarity.
This reverse, so called "swinging", operation of the motor, does not allow a high speed rotation of the motor, as set forth earlier.
To explain the operation of this type of motor reference will be made to a coupling coefficient .gamma.. This coefficient is defined by the following relationship: ##EQU1## where n is the number of turns of coil 6, .phi. is the flux due to the rotor magnet flowing through the coil, and .alpha. the angle of rotation of the rotor. Of course .alpha. is a sinusoidal functon of angle .alpha.: EQU .gamma.=.gamma..sub.0 .multidot.sin .alpha.
.gamma..sub.0 can be calculated from the geometric characteristics of the motor, from the magnetic characteristics of the permanent magnet of the rotor, and from the number of turns of the coil. A value .gamma..sub.0 can therefore be found for every motor.
This value of .gamma..sub.0 also makes it possible to define for the motor a synchronism frequency corresponding to the highest rotational speed that the motor can reach. This synchronism frequency, F.sub.s, is obtained by the following formula: ##EQU2## where U.sub.i is the true induced voltage.